This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
In wireless communication, different collections of communication protocols are available to provide different types of services and capabilities. High speed packet access (HSPA) is one of such collection of wireless communication protocols that extends and improves the performance of existing UMTS (universal mobile telecommunications system) protocols and is specified by different releases of the standard by the 3rd generation partnership project (3GPP) in the area of mobile network technology. The other non-limiting example wireless communication protocols are long term evolution (LTE), global system for mobile (GSM) and worldwide interoperability for microwave access (WiMAX).
Current and future networking technologies continue to facilitate ease of information transfer and convenience to users. In order to provide easier or faster information transfer and convenience, telecommunication industry service providers are developing improvements to existing networks. Carrier aggregation technology has drawn considerable attention in, e.g., HSPA and LTE.
In Release 8 (Rel-8) of HSPA standardization of 3GPP, dual-carrier HSDPA (high speed downlink packet access) was specified by introducing dual-carrier operation in the downlink on adjacent carriers. In an example embodiment, dual-carrier HSPA may be used where a MAC (medium access control) scheduler may allocate two HSPA carriers in parallel and double the communication bandwidth. Besides the throughput gain from double the bandwidth, some diversity and joint scheduling gains may also be expected. This can particularly improve the quality of service (QoS) for end users in poor environment conditions that cannot be gained from other techniques. Similar idea is under consideration in the enhanced LTE technology called LTE-Advanced. Via this technology LTE is expected to improve end-user throughput, increase sector capacity, reduce user plane latency, and consequently offer superior user experience with full mobility.
In Release 9 studies of the HSPA track, a study item termed DC-HSUPA (dual-cell high speed uplink packet access) for uplink dual carrier UE (user equipment) operation has been launched. In DC-HSUPA, the UE may be assigned one or two uplink carriers for data transmission if the UE is dual carrier capable. As compared to downlink dual-carrier operation, where the UE is required to receive the dual-carrier transmission transmitted by the Node B or base station, in the uplink the UE is power limited and thus it needs to share its transmission power among the carriers if it transmits on both carriers simultaneously.
Power amplifier (PA) of the transmitter is non-linear, which causes distortion that degrades the error vector magnitude (EVM) and increases spurious emissions (SE). Signals that have higher peak to average ratio (PAR) will also have a higher linearity requirement for the PA. There are two possibilities to meet the higher linearity requirement: either the PA is designed to be more linear or the operating point of the existing PA has to be set so that the signals do not get distorted. As the PAs become more expensive from the cost and power consumption point of view when the linearity of the PA is increased, it may be more desirable to use the existing PA designs. The distortion, and thus EVM and SE, of the PA may be controlled by adjusting its operating point. Typically, when PAR of the base band signal increases the operating point of the PA has to be adjusted towards more linear region in order to maintain EVM and adjacent channel leakage ratio (ACLR). This adjustment can be done by increasing an output back-off of the PA.
An example of the increase of PAR is in an HSPA communication, when the HS-DPCCH (high speed dedicated physical control channel) and E-DPDCH (E-DCH dedicated physical data channel, wherein E-DCH stands for enhanced dedicated channel) channels are multiplexed into the Release 99 channels. For high power levels this may cause the power amplifier to work in non-linear region, thus may increase ACLR and spectrum mask leakage. In order to tackle this problem and to enable to use PAs that have been designed for the Release 99, the standard allows the UE to reduce the maximum transmit power when HS-DPCCH and/or E-DCH are present.
The calculation of maximum power reducation (MPR) involves the cubic metric (CM). The CM value approximates the 3rd order non-linearity of PA and enables to generalize the amount of PA back-off allowed to fulfill the ACLR requirements.
The cubic metric may be computationally complex and it may need to be calculated every time the channel gain factors change. For example, in HSPA, the calculation of CM for every transmission time interval (TTI) would be enough. However, because the HS-DPCCH transmission may change on every slot, CM may need to be determined for every slot. Furthermore, if the E-DPDCH scaling occurs, the cubic metric may need to be re-calculated within the current slot before the data is to be transmitted. A method that calculates CM for single band or carrier scenario is described in the related application with U.S. patent application Ser. No. 12/453,433, titled “Apparatus, system, and method for calculating a non-linearity metric”. This method utilizes the channel gain factors, determines the constellation points (i.e., the signal states to be transmitted) and calculates the cubic metric on the basis of the constellation points.